Method of making a solid-state superconducting electromagnetic radiation detector

ABSTRACT

Method for making a solid-state superconducting electromagnetic radiation detector by the steps of dividing aluminum into tangled, irregular, clusters of small randomly arranged oxide coated aluminum particles that are compressed and shaped into a self-sustaining solid mass. Connecting leads to the mass and cooling the mass to superconducting temperatures provides a transducer-detector that produces an output voltage in response to electromagnetic radiation that is incident on the surface of the mass over a broad band of frequencies.

United States Patent Strongin et al.

[ 1 July 16, 1974 METHOD OF MAKING A SOLID-STATE SUPERCONDUCTING ELECTROMAGNETIC RADIATION DETECTOR Inventors: Myron Strongin, Center Moriches;

Anand M. Saxena, Upton; Jack E. Crow, Bellport, all of NY.

The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

Filed: Apr. 4, 1973 Appl. No.: 347,756

Assignee:

U.S. Cl 324/43 R, 29/599, 307/306 Int. Cl G0lr 33/02 Field of Search 324/43 R; 29/599; 307/306 References Cited UNITED STATES PATENTS 12/1966 Rosi et a1 29/599 3,325,888 6/1967 Weinig et al 29/599 FOREIGN PATENTS OR APPLICATIONS 1,196,788 7/1970 Great Britain 307/306 Primary ExaminerR0bert .1. Corcoran Attorney, Agent, or Firm-John A. Horan; Leonard Belkin; Cornell D. Cornish {5 7] ABSTRACT Method for making a solid-state superconducting electromagnetic radiation detector by the steps of dividing aluminum into tangled, irregular, clusters of small randomly arranged oxide coated aluminum particles that are compressed and shaped into a self-sustaining solid mass. Connecting leads to the mass and cooling the mass to superconducting temperatures provides a transducer-detector that produces an output voltage in response to electromagnetic radiation that is incident on the surface of the mass over a broad band of frequencies.

8 Claims, 9 Drawing Figures SHED 1 0i 3 VACUUM SYSTEM ngmgn JUL 1 6I974 3; 824.457

sum 2 or 3 lll l I l l NUMBER (ARBITRARY UNITS) L 1 l--. O 66 I32 330 495 660 PARTICLE DIAMETER (K) Fig 2 BACKGROUND OF THE INVENTION This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.

In the field of physics it is advantageous to detect low energy electromagnetic radiation over a broad band of frequencies from about 200 KHz upwardly. Various means and methods have been proposed or used for the desired detection, such as the detector used for radio astronomy at various laboratories and universities throughout the United States, but the receivers used heretofore have been large and cumbersome or they have been difficult or expensive to build or aim. It is additionally advantageous to provide compact microwave detectors and a method of making the same in "a variety of shapes and sizes for the sensitive detection of microwaves over a broad band of frequencies.

It is an object of this invention, therefore to provide SUMMARY OF THE INVENTION This invention provides an easy to build and operate radiation detector and a method of making the same in a variety of shapes and sizes for detection over a broad band of frequencies above 200 KHZ wherein the detector has a major amount of superconductor particles in a minor amount of an insulating sea. More particularly, this invention provides a method of dividing a superconductor into irregularly shaped randomly arranged superconductor particles having a'thin insulating coating, and compressing the same into the desired shape and size for the sensitive detection of electromagnetic radiation at operating temperatures over a broad band of frequencies above 200 KHz. To thisend, in one aspect the compressed mass forms a plurality of continuous intertwined, non-uniform cr0sssection, superconductor filaments having a critical current density of the order of 10' to 10' amps per sq. centimeter, wherein small cross-section weak mechanicalelectrical links having a diameter of below about 100 A connect to larger cross-section strong mechanicalelectrical links having diameters of 100 600 A that are arranged in continuous intertwined chains of alternate periodic weak and strong links having a mass resistance of at least 3 X 10 ohm-cm.

In one embodiment, the method of this invention comprises dividing an aluminum ingot into cubes by cutting, cleaning the cubes byetching, evaporating the cleaned cubes in achamber containing a protective at-v mosphere, depositing the evaporated material on the' h p d randomly a ged 1 iS rssomP e n t tangled clusters to interlock them together in a solid ped ma fi s g e ma s b o nsstin l ad thereto, and cooling the finished solid to operating tern,-

' Pe e W t the Pr per s le t of steps and s n itions, as described in more detail hereinafter, the desired detector and detection is achieved.

The above and further novel features and objects of this invention will be apparent from the following detailed description of one embodiment when read in connection with the accompanying drawings, and the novel features will be pointed out in the appended claims.

BRIEF DESCRIPTION IN THE FIGURES FIG. 1 is a partial cross-section of the apparatus of this invention for performing the method thereof; FIG. 1a shows the detector;

FIG. 2 is a graphic representation of the number of particles vs particle diameter of this invention that were employed in one example of the detector of this'invenn;

FIG. 3 is a partial three-dimensional view of one embodiment of adetector made by the method of thisinvention and the apparatus of FIG. 1; FIG. 3a is a partial three-dimensional view of the filaments of this invention; and 3b is a partial three-dimensional view of the intertwined, interlocked chains and filaments of this invention, having proximate boundaries .th'erebetween;

, FIG. 4 is a partial detailed view of the particles and clusters of the detector of FIG. 3;

cooled to operating temperatures in accordance with this invention; I v

FIG.16 is a graphic illustration of the resistance of the detector of FIG. 3 vs temperature.

DETAILED DESCRIPTION OF ONE EMBODIMENT This invention is useful in making inexpensive, compact, easy to fabricate and operate radition detectors in a variety of shapes and sizes for the sensitive detection of electromagnetic radiation over abroad range of frequencies. These frequencies, for example, are above 200 KHz. As such this invention is useful in the field of radioastronomy, but it is also has a wide utility in any application where such detection is required, since this invention provides a detector that is sensitive to weak radio or other low power level electromagnetic radiation. Thus, this invention is useful in the field of physics generally, as well as in the field of radioastronomy particularly, as understood hereinafter from the detector 9 of FIG. la.

Referring to FIG. 1, a glass bell jar 10 is sealed from the ambient atmosphere and evacuated through a suitable exhaust 12 having a conventional vacuum pump (not shown) after a sample of an aluminum cube 14 is placed in filament 26 inside glass sphere 16, which is located inside the bell jar 10. After evacuation, the bell jar 10: is pressurized to a pressure indicated by gauge 18 from a conventional protective I-Ie source through pipe 22 and valve 24. A current source connected to filament 26 through leads 28 then boils off the aluminum fromthe sample 14, which was placed inside of the filament. FIG. 2 shows-the particle sizes produced.

The aluminum cube is evaporated from the filament 26 and deposited in irregularly shaped, randomly arranged, aluminum particles on the inside of the glass sphere l6, and the described aluminum particles are removed from the glass sphere and formed into tangled, irregularly shaped, randomly arranged clusters. The particles are then coated completely and uniformly with A1 by exposure to 0 To this end, the particles are brushed from glass sphere 16 into a suitable containerin an oxygen containing ambient that forms the A1 0 coating on the particles. The ambient air is used for this. The number of particles vs average particle size recovered is illustrated by FIG. 2. Herein, the sizes used are the maximum cross-sectional dimension.

As seem in FIG. 3, the coated particles 32are collected randomly in a mold 33 cylindrically shaped and compressed at 1,0005,000 psi, where, as illustrated in FIGs. 3 a, 3b and 4, there are formed a solid mass 34 consisting of interlocked chains 47 of non-uniform cross-section aluminum filaments 41, having boundariesSl therebetween, as described in more detail hereinafter. The relative size of the interlocked particle clusters 36 and the interlocked particles 32 are illustrated in FIG. 4. Suitableleads 37 applied to the opposite ends of the solid mass 34 provide the finished detector 9, which is operable when cooled in a cryostat 39, as shown in FIG. 1a. Means 40 send the radiation, and volt meter v shows the voltage. I

It will be understood from the above that the process of this invention compresses the particles and clusters into a plurality of continuously interlocked and intertwined, non-uniform cross-section, aluminum filaments 41. These filaments have a critical superconducting transition current density of about 10 to amps per square centimeter. As shown in FlG .3a the filaments 41 have weak mechanical and electrical links 43 forming aluminum filaments below about 100 A in cross-section connecting larger cross-section aluminum filaments between about 100 600 A arranged as strong links 45 in continuous intertwined chains 47 of alternate periodic strong and weak links.For purposes of this application, weak links are aluminum filaments having a cross-sectional diameter of below about 100 A, and strong links are like aluminum filaments having a cross-sectional diameter of about 100 600 A.

In operation, the voltage output of the solid mass 34 vs temperature is illustrated in FIG. 5. The solid mass 34 is cooled to within l of the A1 T and'the leads 37 are connected to conventional circuitry, such as a voltmeter, tor determining the voltage output from the detector 9 in response to incident electromagnetic radiation on the surface of the solid mass 34. FIG. 6 illustrates the resistance vs temperature of the detector of FIG. 3.

Examples of the method and apparatus employed in making one embodiment of the detector 38 of FIG. 3 are -as follows:

EXAMPLE I.

A standard water-cooled carborundum saw divided an aluminum ingot into small cubes about 2 3 millimeters ona side. Etching in 70 pts H;PO,, 12 pts CH COOH. 3 pts HNO and pts H O then removed any surface contamination from the aluminum (purity 99.9 Percent).

Thealuminum was placed in a helical tungsten filament inside a vacuum chamber which was evacuated to net. The evaporation and depositing was continued until all the aluminum in the etched cube has boiled off, and then the system was allowed to cool to room temperature before the vacuum (vacuum bell jar) was vented to the atmosphere;

The venting to the oxygen containing ambient coated the deposited particles with a thin minority layer coating of an insulating oxide with a major portion of the aluminum remaining. These coated particles were collected into tangled, irregular, randomly arranged clusters by brushing or scraping them into a collecting container. The particles were then transferred to a mold where shaping and linking of the clusters was achieved by compression of about 1,000 psiinto the desired shaped solid mass containing the plurality of intertwined, non-uniform cross-section, superconductor filaments having a critical superconducting transition current density of about 10' to 10" amps per square centimeter, wherein weak links below about A connected strong links between about 100 600A arranged in continuous intertwined and interlocked chains of alternate periodic strong and weak links, as determined visually.

Electrical leads were attached either by silver painting or pressing to finish the detector. Upon cooling the finished detector in a cryostat to an operating temperature at least as low as 1.3 l.4 K (the critical superconducting temperature T, of aluminum .being 1.19 K) and exposing the surface of the shaped, solid, cooled mass to electromagnetic radiation of 200 KHZ or higher, a DC. voltage was detected across the leads. Actual voltages were detected between 200 KHz and 4 GHz below 1.3 1.4 K. The radiation was applied in this example by a coil around the solid mass of cylindrical sample with theends of the coil connected'to a continuously variable frequency and amplitude rf oscillator.

. EXAMPLE ii The steps and conditions of Example I were repeated using various gas pressures using both helium and resid- Example 11! The steps and conditions of Example I were repeated using various gases such as Ar, Xe. Kr and lighter gases during the evaporation step to study the effect of different molecular weight or size of the gas molecules on particle size. For comparable conditions, He gas gave the smallest particles and best results, and was determined to be the best gas. although Ar. Xe or Kr could also be used alone or in combination.

Example IV The steps and conditions of Example I were repeated at various substrate and collection surface temperatures and it was found that satisfactory results, comprising the desired particles, particle sizes, and clusters were obtained at incrementally increasing filament temperatures starting at the melting temperature of aluminum with the collection surface temperature approximately equal to room temperature of about 72 F. The currents used to melt the aluminum cube were typically about 100 Amps a.c. Upon melting at 100 Amps, the aluminum was retained along the filament by surface tension. The current was then further increased about 1 amp every l-2 minutes to achieve the evaporation. This evaporation took about 2-5 minutes for an aluminum cube having a mass of 2 3 grams.

Example V The steps and conditions of Example I were repeated at slow evaporation rates. It was determined that after the aluminum melting point was reached, that by not increasing the current further the result was that there was too low of an evaporation rate to form aluminum particles, because of the presence of small reactive gases in the bell jar. For example, an evaporation rate of more than /2 hour had the effect of not producing particles, i.e., the aluminum coated the inside of the glass sphere on which it was condensed.

Example Vl .T he steps and conditions of Example I were repeated for various compaction pressures to produce inhomogeneously interlocked and compacted clustersandparticles. It was found that the resistance of the aluminum superconductor decreased at about 1.3 K as the compaction pressures increased from 500 to 5000 psi. In this example, at 1.3K the resistivity of the compacted sample was ohm-cm, 3 X 10 ohm-cm, 100 ohm-cm at compaction pressures or 400 psi, 1,000 psi and 5,000 psi. At the superconducting critical transition temperature T of the sample, the optimum resistivity was 103 300 ohms-cm.

Example VII The step and condition of, Example I were repeated to show the effect of decreasing the temperature of the cryostatically cooled'cylindrical detector. It was found that for a given radiation input incident on the outside cylindrical surface of the detector that an increasing positive voltage output was produced, which increased as the temperature decreased from about 1.35 K to about 1.0 K as shown in FIG. 5; also, the voltage output level was substantially constant from MHz to at least 400 MHz. A slightly increasing voltage output was produced by incident radiation from 200 KHz to as high as 8 CH2.

Example VIII The steps and conditions of Example I were repeated in various molds without decreasing the sensitivity of the detector produced. Cylindrical detectors of various sizes were made, but many other rounded or flat shapes, such as spherical, square, disc, or rectangular, could be satisfactorily made and used.

It was found that, by compacting 300 A A1 particles in the form of clusters the composite material was ompos d o greater that jun tions th t ed coherently when exposed "to rf surface radiation. In these large arrays with a large number of junctions, a steady state dc voltage ofup to 200 iV was observed from the leads connected to the compressed solid mass or particles even with zero dc bias boltage applied, to the detector. This was in contrast to the behavior of a single junction with a zero bias voltage present, where it was found that the single junctions switched between different quantum stages. FIG. 6 shows the resistive transition of thearray of particles in the detector. The transition has a long tailfthis behavior is ordinarily characteristic of weakly linked cooperative systems, which are extremely current sensitive It was also found that the dc output voltage from the detector varied as a function of frequency for constant amplitude rf incident energyQBy effectively changing the coupling of the radiation to the detector with a tuning stub, which was similar in function to the ones shown in FIGs 13-16 U. S. Pat. No. 3,530,332, by Giordano, or by changing the frequency tuning, the voltage sign could be made to change from plus to minus.

The radiation power input in this example followed that of the others, where this input was less than a microwatt. In the cylindrical detector of this example, where the cylinder was l/4 inch in diameter, the response was good for a number of tests from 1 MHz to 4 Gl-lz, although higher frequencies or lower frequencies down to about 200 KHz were also possible. The

Example IX A detector was fabricated in the same way as described in Example 1, except that after the particles were removed from'the bell jar and before they were compressed, any existing particle clusters were broken up. The breaking up of the particle clusters was achieved by passing a colloidal solution of the particles suspended in methanol through a closely fitted piston and cylinder. The piston was rotated at a high velocity and the cylinder remained stationary as the colloid was forced between the piston and cylinder; the resulting shear waves produced in the liquid broke the particle clusters. The separated particle were collected by allowing the methanol to evaporate leaving the particles as a residue. The individual particles were then treated similarly to Example I with the additional exception that a larger pressure was required in the molding process to attain the desired resistivity of the detector. This process had the advantage that the higher pressure used in the molding process resulted in a sample which was less susceptible to damage during handling.

In the above examples it was not completely understood by the large cross-section, inhomogeneous interlocking of irregularly shaped particles and clusters under compression produced the desired results without biasing, not why theweak small cross-section, filamentary, superconductor links between the particles didnt cancel but acted synchronously and coherently with enhanced sensitivity as they did, but is is theorized that the inhomogenous junction array internally was shaped to form a coherent cooperative system of Jo- "sephson and/or other junctions independently of the been made by the described process with individual particle clusters 'where the individual particles were about300 A across to provide 10 or more junctions on the macroscopic scale. Besides the enormous structure in the dT/dV vs V characteristic, a striking property of the superconducting state was that when rf radix ation'impinged on unbiased samples, a constant dc voltage of up to 200 uV or more appeared that was stable intime and dependent on the frequency range of at least from 1 MHz to 4 G Hz, in contrast to theswitching of single junctions in sign. and voltage output magnitude.

Looking at the detectoras an alloy, the superconducting aluminum provided a majority aluminum constituent in sea. of, aluminum oxide forming a normal resistance insulator matrix of aluminum oxide forming a minority constituent and proximate boundaries 500 A across the'boundaries. Thus, as shown in H6. 3b, a plurality ofinterstitial boundaries 51 could exist between the continuous, intertwined chains of non-uniform cross-section, superconductor filaments 41, and these boundaries could in some cases individually conduct by tunneling.

While the above has described the use of aluminum particles having an oxide coating thereon for forming the detector of this invention, it will be understood that other superconducting metals having oxide I coating thereon can be used. For example, tin particles having a tin oxide coating thereon can alternately be used. In this case the operating temperature would be higher than with the Al particles due to the fact that the superconducting transition temperature of the array would be near -3.7 K. Other examples, would employ oxide coated noibium particles, where the superconducting transition temperature, i.e., the T is near 9 K.

This invention has the advantage of providing a detector and method for making it for detecting radiation from 200 KHz upwardly over a broad band offrequencies up to at least 8 GHz at a wide' variety of amplitudes as weak as those contemplated for radioastronomy purposes. Moreover, the detector of this invention is small, compact, easy to assemble andoperate,'and has any ordinary shape that is desired, such as cylindrical, rounded, rectangular,disc-shaped, etc.

What is claimed is:. e i

1. Method for making a solid, solid state, electrically non-biased, mechanically self-sustaining nonadjustably clamped, superconducting electromagnetic radiation detector that is substantially without intersticial spaces while being sensitive at between 1.3 to 1.4 K to the presence of an electromagnetic radiation input of at least 200 kHz, comprising the steps of:

a. collecting irregularly shaped, randomly arranged resistive particles of average cross-section in the range of 100 600 A of aluminum having a purity of 99 percent coated with A1 0 into tangled, irregular, randomly arranged resistance clusters;

b. compressing, the resistive clusters at sufficient pressure resistively to interlock the particles so as to produce resistive mechanical and electrical links between adjacent particles having a resistance of 3 X 10 ohm-cm and a critical superconducting tran- 'sition current density of between about 10' to 10 amps/cm", said links averaging predominantly 100 A orbelow in cross-section said compressed particles forming a solid mechanically self sustaining mass upon removal of the pressure;

c. connecting electricaloutput leads to separated portions of the compressed clusters; and

d. cooling the compressed clusters to a temperature of within 1K of the critical superconducting transition temperature T of said aluminum.

2. The method of claim I'in which said clusters are evaporated aluminum on the inside surface of a container holding a helium atmosphere containing residual oxygen. I e

4. The method of claim 1, in wh-ichsaid step of compressing said tangled, irregular, randomly arranged clusters interlocks the same in a multiplicity of continuous interlocked chains of periodic alternate strong'and weak links forming non-uniform cross-section superconductor 'tilame nts.

5. The method of claim 1, including the step of detecting a voltage output from said solid mass in response to an electromagnetic radiation field at thesurface of the mass at a frequency of at least 200 KHz where the mass is cooled to an operating temperature within 1,K of the temperature where the resistance rapidly decreases to the superconducting transition temperature of the aluminum particles.

6. The method of claim 1 in which said compressing is in a mold under a compactingpressure of up to about 5,000 pounds per square inch that formsa cylindrical body in said mold.

tance component.

is x: it 

1. Method for making a solid, solid-state, electrically nonbiased, mechanically self-sustaining non-adjustably clamped, superconducting electromagnetic radiation detector that is substantially without intersticial spaces while being sensitive at between 1.3 to 1.4 K to the presence of an electromagnetic radiation input of at least 200 kHz, comprising the steps of: a. collecting irregularly shaped, randomly arranged resistive particles of average cross-section in the range of 100 - 600 A of aluminum having a purity of 99 percent coated with Al2O3 into tangled, irregular, randomly arranged resistance clusters; b. compressing the resistive clusters at sufficient pressure resistively to interlock the particles so as to produce resistive mechanical and electrical links between adjacent particles having a resistance of 3 X 103 ohm-cm and a critical superconducting transition current density of between about 10 5 to 10 6 amps/cm2, said links averaging predominantly 100 A or below in cross-section said compressed particles forming a solid mechanically self sustaining mass upon removal of the pressure; c. connecting electrical output leads to separated portions of the compressed clusters; and d. cooling the compressed clusters to a temperature of within 1*K of the critical superconducting transition temperature Tc of said aluminum.
 2. The method of claim 1 in which said clusters are compressed at a pressure in the range of from 1,000 to 5,000 psi.
 3. The method of claim 1, in which the collecting is by the evaporating of said aluminum from a heated tungsten filament followed by the condensation of the evaporated aluminum on the inside surface of a container holding a helium atmosphere containing residual oxygen.
 4. The method of claim 1, in which said step of compressing said tangled, irregular, randomly arranged clusters interlocks the same in a multiplicity of continuous interlocked chains of periodic alternate strong and weak links forming non-uniform cross-section superconductor filaments.
 5. The method of claim 1, including the step of detecting a voltage output from said solid mass in response to an electromagnetic radiation field at the surface of the mass at a frequency of at least 200 KHz where the mass is cooled to an operating temperature within 1* K of the temperature where the resistance rapidly decreases to the superconducting transition temperature of the aluminum particles.
 6. The method of claim 1 in which said compressing is in a mold under a compacting pressure of up to about 5,000 pounds per square inch that forms a cylindrical body in said mold.
 7. The method of claim 3 in which said particles are evaporated and condensed on a surface at an average size of between about 100 to 600 Angstroms.
 8. The method of claim 7 in which an A12O3 coating is formed on the aluminum particles by surrounding said particles with an oxygen containing ambient to produce a coating thereon, said coated particles being compressed so that upon said compression said coated particles provide a mass containing a major amount of a first aluminum superconductor component in a matrix of a minor amount of a second Al2O3 normal resistance component. 